refactor: 增加 null 保护防止报异常

Author: ArgoZhang

src/BootstrapBlazor/Services/CacheManager.cs

@@ -246,7 +246,7 @@ private static JsonLocalizationOptions GetJsonLocalizationOption()
                     {
                         value = kv.Key;
                     }
-                    return new LocalizedString(kv.Key, value!, false, typeName);
+                    return new LocalizedString(kv.Key, value ?? "", false, typeName);
                 });
 #if NET8_0_OR_GREATER
             return items?.ToFrozenSet();